The present invention relates to an automatic process controller and method employing a closed circuit TV camera to monitor the progress of the process. More particularly, in the exemplary embodiment disclosed herein, the present invention relates to a real time automatic process for forming end-opening blisters during the manufacture of thermometers.
In the manufacture of thermometers, a glass rod is cut to a size somewhat greater than the length of two thermometers. The glass rod is then heated and a large blister is blown approximately in the center of the rod using compressed air. After the rod has cooled the rod is cut in half through the center of the blister. In this way the large end opening is formed in the thermometer. A bulb is fastened to the end opening to seal that end and the thermometer is filled with mercury. After degassification of the mercury, the thermometer is calibrated, the excess mercury removed and the other end sealed.
One of the more critical steps in the manufacture of a thermometer involves the forming of the large or end-opening blister. Traditionally, this blister has been blown by an operator while observing an optically magnified image of the blister and controlling the compressed air supply by, for example, using a foot-operated treadle. Although most operators had fairly good eye-foot coordination, the defect rate for thermometers due to unsatisfactory end opening blisters was undesirably high. This was due in part to the fact that there are tremendous variations, even within the same lot of glass, in how readily the blister can be blown. These differences result from differences in the thicknesses of the glass rods as well as differences in the bore diameters thereof. Moreover, movements in the manufacturing area can cause drafts which vary the flames being used to heat the glass rods prior to forming the end opening blisters therein.
One of the complicating factors associated with blowing the large or end-opening blisters is the phenomenon of "coast" or "drift". Even though the end-opening blister is blown in the glass rod after it has been removed from the heat and has started to cool, the blister does not stop growing immediately after the compressed air is turned off. Rather, the blister continues to expand or "coast". Moreover, the rate and extent of coast varies significantly from piece of glass to piece of glass, even within a single batch. Accordingly, even with the best of eye-foot coordination it was impossible for an operator to predict the amount of coast to be expected with a given piece of glass. Hence, the undesirably high rejection rate due to unsatisfactory end opening blisters.
The present invention solves the problems associated with blowing end-opening blisters in thermometers. In accordance with the present invention, a closed circuit TV camera is focused on a magnified optical image of the blister. The analog video information is analyzed to detect the leading and trailing edges of the blisters. Digital numbers corresponding to the leading and trailing edges are produced and fed to a computer where they are analyzed and process control signals are developed based thereon.
More particularly, the digital numbers are analyzed to determine the actual size of the blister. In addition, the "coast" associated with the blister is calculated. The difference between the desired blister size and the actual blister size is determined and the "coast" substracted therefrom. A sufficient quantity of compressed air is then supplied to the rod to cause the blister to grow to one-half of the difference between the desired size and the actual size, minus the "coast". The air is then turned off, the edges of the new blister determined and a new "coast" calculated. The difference between the desired blister size and the new blister size is again computed and the new "coast" subtracted therefrom. Enough compressed air is supplied to the blister to cause it to grow by one-half of the remaining distance, minus the new coast.
The above-described iterative process is repeated many times a second. In this manner uniform end-opening blisters are formed and the rejection rate due to improper end-opening blisters is greatly diminished.
While the particular embodiment of the invention is disclosed in connection with the formation of end-opening blisters in thermometers, it will be appreciated by those skilled in the art that the automatic process controller of the present invention has wide application. It may be used, for example, to control the size of a workpiece being turned on a lathe. Broadly, the present invention can make either relative or absolute comparisons between static shapes or between objects of varying size and their actual location versus their desired location, the only requirement being that the object be susceptible to view by a TV camera.